Wake of inertial waves of a horizontal cylinder in horizontal translation
Résumé
We analyze theoretically and experimentally the wake behind a horizontal cylinder of diameter $d$ horizontally translated at constant velocity $U$ in a fluid rotating about the vertical axis at a rate $\Omega$. Using PIV measurements in the rotating frame, we show that the wake is stabilized by rotation for Reynolds number $\mathit{Re} = Ud/\nu$ much larger than in a non-rotating fluid. The limit of stability is $\mathit{Re} \simeq (275\pm25)/\mathit{Ro}$, with $\mathit{Ro} = U/2\Omega d$ the Rossby number, indicating that the stabilizing process is governed by Ekman pumping in the boundary layer. At low Rossby number, the wake takes the form of a stationary pattern of inertial waves, similar to the wake of surface gravity waves behind a ship. We compare this steady wake pattern to a model assuming a free-slip boundary condition and a weak streamwise perturbation. Our measurements show a quantitative agreement with this model for $\mathit{Ro} \lesssim 0.3$. At larger Rossby number, the phase pattern of the wake is close to the prediction for an infinitely small line object. However, the wake amplitude and phase origin are not correctly described by the weak-streamwise-perturbation model, calling for an alternative model for the boundary condition at moderate rotation rate.